Because a child who doesn't find pain unpleasant is really, really handicapped, even in the modern world. The people who founded A Gift of Pain had a daughter with pain asymbolia who is now mostly blind, amongst other disabilities, through self-inflicted damage. I'm not sure whether leprosy sufferers have the no-pain or no-suffering version of pain insensitivity (I think the former) but apparently it's the reason they suffer such damage.

This book seems to be a useful source for people considering the question of whether pain could be improved.